A permanent magnet synchronous motor (PMSM) can be controlled with a location sensor or without a location sensor. For the control method with the location sensor, a magnetic field location of the synchronous motor is detected by the location sensor, whereas for a control method without the location sensor, a rotor location is estimated by a location estimation module. The out-of-step of the synchronous motor is defined with respect to a synchronization, i.e., a rotating magnetic field of the stator and the magnetic field of a rotor magnet do not rotate synchronously. In the PMSM system, the out-of-step occurs due to reasons such as high system load and reduced motor performance. When the motor is out of step, the rotating speed of the motor is generally uncontrollable, which will cause serious losses and safety risks. Thus, there is a need to check the out-of-step of the synchronous motor precisely and timely.